The present invention relates to an injection molding method for manufacturing a thermoplastic part with thick and thin wall sections. The method inhibits the formation of surface defects, such as sink marks, which are typically present in such complex parts when manufactured by conventional injection molding method.
In the case of a large-sized part such as a housing for a TV set or a bumper for an automobile, the part must have structural strength as well as excellent surface appearance and dimensional accuracy. In order to obtain structural strength suitable for large parts, it is preferable that the part be designed to include thick wall sections which will reinforce the strength of the part. Hence, the structural integrity of the molded part will not be solely dependent upon the strength of the resin. These conflicting requirements often result in the sacrifice of appearance for strength or vice versa due to the dynamics of the conventional injection molding process.
When a part having both thick and thin wall sections is injection molded, the molten resin in the interior of the mold cavity is cooled and solidified at a slower rate than the exterior surface. Due to this delay of cooling and solidification, the volumetric shrinkage of the resin is likely to be accumulated in the more interior sections of the part. The accumulation of the volumetric shrinkage often causes the formation of sink marks which especially occur on the surface of the thick wall sections, deformations and other defects (which are hereinafter referred to as sink marks) of the part. Consequently, it is difficult to obtain a high quality surface in the molded part.
Due to this phenomenon, in a conventional injection molding process, parts are designed to avoid thick wall sections. When a part with thick wall sections must be manufactured by the conventional injection molding process, a dwelling step immediately after the injection of a molten resin into the cavity of a mold is performed. In the dwelling step, a dwelling pressure is applied to the molten resin existing in the cavity of the mold to maintain the packing density of the injected molten resin. However, when a part has a complicated configuration, a gate sealing precedes the completion of dwelling. As a result, sufficient dwelling pressure cannot be applied to the molten resin at a position far from a gate. The resulting part will not have a satisfactory surface appearance.
Japanese Patent Application Publication 61-53208, Japanese Patent Application Laid Open 63-268611 and Japanese Patent Application Laid Open 64-63122 disclose methods using a high-pressure gas as an auxiliary means of aiding the conventional injection molding process to inhibit the formation of sink marks. These methods comprise a two step process to inhibit surface defects in a molded part. In the first step, the molten resin, in an amount insufficient to fill the cavity of the mold is injected into the cavity. At the same time or after, high-pressure gas is injected into the molten resin to form a hollow portion through a resin flow passage. In the second step, the application of dwelling pressure derived from the high-pressure gas through the hollowed resin flow passage to the interior of the molten resin is maintained so that the molten resin in the cavity is cooled and solidified against the inner wall of the mold to inhibit the formation of surface defects.
However, when a part having a complicated configuration is to be manufactured, the thick wall portion(s) connected with a gate are likely to act as flow leaders for the molten resin, so that the supply of the molten resin to each portion of the mold cavity becomes unbalanced. The result of this unbalance is air traps and flow marks which result in surface defects on the part. In extreme cases, the high-pressure gas may break the surface layer of the solidifying resin part so that the injection molding cycle itself is interrupted and a defective part is manufactured. Moreover, there is a restriction on the design configurations of parts to which these methods are applicable. If a hollow portion sufficient to inhibit the formation of sink marks cannot be formed in the mold cavity, the formation of sink marks on the surface of the part will result. Thus, these methods are not satisfactory for insuring parts with superior surface appearance.
Japanese Patent Publication 48-41264 discloses another injection molding method using high-pressure gas as auxiliary means, when an article having a thick wall throughout the part is to be manufactured. In this method, after molten resin is injected into the cavity of a mold, a gas nozzle is directly projected into the molten resin in the cavity. High-pressure gas is supplied through the gas nozzle into the molten resin to perform the aforementioned first step while forming a hollow portion in the molten resin. This avoids the first step problems enumerated above. However, the configuration of a part to which this method is applicable is limited to uniformly thick wall sections. Moreover, this process requires a reciprocal carrying mechanism for inserting the gas nozzle into the molten resin during injection molding and withdrawing the gas nozzle from the solidified part. This method lacks in practicality, since it requires a driving mechanism with a withdrawing power larger than a constraining force derived from the shrinkage of the resin during solidification t remove the gas nozzle from the part.
Besides, these methods have significant problems such as the danger which accompanies the use and handling of high-pressure gas.
Japanese Patent Publication 61-9126 discloses the method wherein the cavity of a mold is filled with a molten resin, and the thick wall portion of the part is then pressed by a gas pressure from a position corresponding to the back side of the part. However, the direct application of the gas pressure causes the formation of irregular sink marks on the back surface of the part resulting in an unsatisfactory surface appearance.
When a resin such as polycarbonate or polymethyl-methacrylate, which set up quickly, is injection molded under conditions that permit the surface layer to rapidly cool and solidify, the formed surface layer of the part exhibits higher strength than the force derived from the volumetric shrinkage of the resin. Consequently, voids are formed in the thick wall portion of the part without deformation of the surface layer. In this case, sink marks are not formed on the surface of the thick wall portion.
The formation of these internal voids is regarded as a defect which causes a reduction of the strength in the part. However, this phenomenon of forming voids can be effectively controlled to inhibit the formation of sink marks, as disclosed in Japanese Patent Application Publication 2-13886. In this method, a void control member is projected through the inner surface of a mold into the cavity where sink marks are apt to form on the surface of the part. The void control member promotes the formation of a void in the resin body near the top end of the void control member, thereby inhibiting the formation of sink marks. This method has the advantage that the formation of sink marks can be inhibited by a simple, economical means.
However, the void control means must be made of a material having large heat capacity, so as to be held at a higher temperature in order to spontaneously form voids. Consequently, it is difficult to make the void control members smaller in size. The position where the void control members may be located in the mold are restricted, and the void control members cannot be held at a high temperature under a stable condition. As a result, it is difficult to ensure the thick wall sections of the part will be free of sink marks.
In addition, the positions where sink marks will be likely to be formed are irregularly affected by the molding conditions. For instance, the sink marks are formed at different positions during every molding cycle, even when the same part is manufactured. In this regard, it is difficult to predetermine the position where sink marks will b formed. The size and the configuration of the thick wall portion has an influence on the positions where sink marks are to be formed as well. Furthermore, in some resins, such as polycetal, the voids do not expand to the extent sufficient to compensate for the volumetric shrinkage of the resin, so that the formation of sink marks on the surface of a part cannot be completely eliminated.
An object of the present invention is to overcome the above-mentioned problems which causes defects in a molded part, and to obtain a molded part with excellent surface appearance, free from sink marks, even when the part has wall sections that vary significantly in thickness.
The present invention to realize the above-mentioned object is to induce a void in the portion near the top end of a void inducing member provided at position of each thick wall portion by the application of a gas pressure along the void inducing member, and then to make the void larger by the shrinkage force of the resin, without using a high-pressure gas which causes various defects. The present invention provides an injection molding method for manufacturing a thermoplastic part having thick and thin wall portions while inhibiting the formation of sink marks on the surface of the part. The steps in this process are:
1. providing at least one void inducing member having an acute top end at a position, corresponding to a thick wall portion where sink marks will be easily formed in a state that the acute top end is located in the cavity of the mold,
2. injecting molten resin in an amount sufficient to fill the cavity of the mold,
3. supplying compressed gas along the periphery of the void inducing member to the acute top end, wherein a resin skin layer is formed in contact with the top end of the void inducing member and is penetrated by the compressed gas, and one fine bubble serving as a void nucleus is formed in the molten resin, and
4. cooling and solidifying the resin, such that the void nucleus is expanded to a large void in response to the volumetric shrinkage of the resin by the shrinkage force of the resin during cooling and solidification.
The application of the gas pressure may be started just after the completion of the resin injection into the cavity of the mold or may be started after the dwelling step wherein the thermoplastic resin injected to fill the mold cavity is held at a predetermined pressure for a predetermined period. In this dwelling process, the molten resin is circulated to all parts of the mold cavity and the counter flow of the injected resin from the cavity to reduce the packing density is also inhibited.
After the void nucleus is formed in the molten resin at an inner part near the top end of the void inducing member by the application of the gas pressure, the application of the gas pressure to the void nucleus may be continued until the molten resin, at the part corresponding to the thick wall portion, loses its fluidity. Thereby, the shrinkage force of the molten resin during cooling and solidification effectively promotes the growth of the void nucleus up to a volume sufficiently corresponding to the volumetric shrinkage of the resin.
The mold to be used in this method has a cavity, comprising at least one large space, corresponding to the configuration of the part to be manufactured. The void inducing member is located in the cavity in such a manner that its acute top end is projected into the large space from the inner surface of the mold cavity. The void inducing member has a passage extending longitudinally along its surface for introducing the compressed gas. The void inducing member has the form of a simple pin, and is secured to the mold in the same manner that of other pins. The void inducing member can, thereby, be easily prepared and incorporated in the mold. In addition, the void inducing member may be used as a core pin of a sleeve ejector for ejecting the sleeve of the part from the mold.
The mold cavity is filled with molten resin, and then the gas pressure is applied to the molten resin along the periphery of the void inducing member projected into the mold cavity immediately after the resin injection or after the completion of the dwelling step. When the interior of the molten resin reaches a negative pressure owing to the volumetric shrinkage of the molten resin during cooling and solidification, the compressed gas breaks through the skin layer of the resin being formed around the acute top end of the void inducing member and a fine bubble is induced in the molten resin. The bubble acts as the void nucleus for the formation of the void. The pressure of the compressed gas which is applied is much lower than the above-mentioned conventional methods using high-pressure gas, since the interior of the molten resin is at the negative pressure and the resin skin layer near the top end of the void inducing member is still thin.
Once the void nucleus is formed, a volumetric shrinkage force derived from the cooling and solidification of the resin near the void inducing member is accumulated as negative pressure in the void nucleus, even if the application of the gas pressure is stopped. Consequently, the void nucleus acts as a seed for the growth of the void, and the void continuously becomes larger in volume while introducing the atmospheric gas through a hole formed at the top end of the void inducing member to compensate for the volumetric shrinkage of the resin. Thus, the function of the void to inhibit the formation of sink marks extends not only to the portion of the part near the void inducing member but also to the entire thick wall portion of the part. When the application of the gas pressure is continued, the gas pressure in combination with the volumetric shrinkage force of the resin effectively promote the growth of the void even though the compressed gas is being supplied at a very low pressure. Consequently, the formation of sink marks can be inhibited not only at the thick wall portion but also at an adjacent thick wall portion and at a thin wall portion near the thick wall portion.
The void inducing member has the acute top end which facilitates an accumulation of the gas pressure near the top end of the void inducing member to make the formation of the void nucleus easy. Owing to the acute top end, the formation of the void at a position in the resin can be reproduced with high reliability.
It is not necessary to increase the heat capacity of the void inducing member. On the contrary, the acute tip of the void inducing member has a small heat capacity.
The diameter of the void inducing member is not limited in particular, but preferably 1 to 5 mm to ensure the mechanical strength of the part. Where the acute top end of the void inducing member is inserted into the large space of the mold cavity, it is not necessary to locate the top end at the center of the large space. The projection of the void inducing member from the inner surface of the mold may be shorter in length similar to the diameter of the void inducing member. Consequently, the formation of the skin layer near the top end of the void inducing member is retarded, and the skin layer is held in a state which is easy to penetrate by the application of the gas pressure.
The resulting hole formed in the skin layer is very small in diameter, e.g. 0.5 mm or so, which is barely visible. Consequently, the appearance of the part is not effected by the formation of the hole.
The position for providing the void inducing member is not limited in particular, but may be located any place convenient with respect to the design of the mold. For instance, a sufficient effect is obtained even when the void inducing member is located near the circumference of the large space.
Consequently, the degree of freedom in designing a part is considerably greater than with the void control member disclosed in Japanese Patent Publication 2-13886. In addition, the void inducing member may have any configuration other than a column, as far as the gas pressure can be applied to the part of the resin near the top end of the void inducing member.
The pressure of the compressed gas to be applied is about 5 to 15 kg/cm.sup.2, which is much lower than that of high-pressure gas used as an auxiliary means in the conventional methods. For instance, the conventional injection molding methods using high-pressure gas disclosed in Japanese Patent Publication 57-14968 and Japanese Patent Publication 61-53208 use high-pressure gas compressed up to 150 kg/cm.sup.2 for hollowing the thick wall portion. In the present invention, an air pressure of 10 kg/cm.sup.2 or less will be sufficient in most cases, so that an ordinary gas supply source is usable. The gas flows along the periphery of the void inducing member and reaches the top end of the void inducing member, so that a slight gap is formed between the surface of the void inducing member and the resin skin layer. The slight gap acts as a gas passage to introduce the atmospheric gas into the void after the supply of compressed gas is stopped. As a result, the void inducing member can easily be extracted from the part, since the void inducing member is not captured by the part different from the method disclosed in Japanese Patent Publication 48-41264. Thus, it should be recognized that the method utilized in the present invention attains superior results over the other known methods.